Wireless telephone with frequency inversion scrambling

ABSTRACT

The wireless telephone of the present invention is comprised of a base unit, coupled to a land-line telephone system, and a remote unit that communicates with the base unit over radio frequencies. The remote unit has scrambling circuits (207 and 216) for encrypting voice signals transmitted to the base unit and decrypting voice signals received from the base unit. The base unit also has scrambling circuits (107 and 116) for encrypting voice signals transmitted to the remote unit and decrypting voice signals received from the remote unit. The wireless telephone uses a keypad (222) for initiating a demonstration mode that allows both parties to the conversation to hear the scrambled voice signal.

FIELD OF THE INVENTION

The present invention relates generally to the field of communicationsand particularly to signal scrambling between a base and remote wirelesstelephone.

BACKGROUND OF THE INVENTION

Cordless or wireless telephones are typically home units that consist ofa base unit and one or more remote units. An example of such a cordlesstelephone is seen in U.S. Pat. No. 4,989,230 to Gillig et al., assignedto Motorola, Inc., and is incorporated herein by reference. The remoteunits communicate with the base unit over radio frequencies (RF)typically within a range of 1000 feet of the base unit. The base unit isconnected to the land-line telephone system, thus allowing a remote userto communicate with other land-line telephones.

The problem with this type of system, however, is that the RF signalscan be received by anyone with a receiver tuned to the frequency beingused by the cordless telephone. This makes it difficult to have aprivate conversation using a cordless telephone. Cordless telephonesthat scramble the RF voice signal don't provide for testing thescrambling function to determine if it is working properly. There is aresulting need for a cordless telephone that makes it difficult for theRF voice signals to be listened to and also enable the scramblingfunction to be checked for proper operation.

SUMMARY OF THE INVENTION

The wireless communication system of the present invention is comprisedof a base unit, coupled to a land-line telephone system, and a remoteunit that communicates with the base unit over radio frequencies. Theremote unit is comprised of encryption means, located in the transmitpath of the remote unit, for encrypting voice signals transmitted to thebase unit and decryption means, located in a receive path of the remoteunit, for decrypting voice signals received from the base unit. The baseunit is comprised of encryption means, located in a transmit path of thebase unit, for encrypting voice signals transmitted to the remote unitand decryption means, located in a receive path of the base unit, fordecrypting voice signals received from the remote unit. The wirelesscommunication system has control entry means, located in the remoteunit, for generating a control signal to disable the decryption means inthe remote and base units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a base unit of the present invention.

FIG. 2 shows a block diagram of a remote unit of the present invention.

FIG. 3 shows a flowchart of the process of the present invention.

FIG. 4 shows a block diagram of the scrambler integrated circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The wireless telephone of the present invention uses scrambled RF voicesignals to communicate between the base unit and the remote unit. Bydisabling some of the scrambling circuits in the remote or both theremote and the base, both parties to the telephone call can determinethat the scrambling function is operational.

The block diagram of FIG. 1 illustrates a preferred embodiment of acordless telephone base unit having a control-crypting signals. Thisparticular base unit also includes a speakerphone. FIG. 2 illustrates apreferred embodiment of a remote unit for communicating with the baseunit, the remote also having a controllable scrambling or encryptioncircuit for encrypting and decrypting signals.

Referring to FIG. 1, the base unit is comprised of an antenna (101) thatis coupled to the receive and transmit paths of the base. The receivepath is comprised of a bandpass filter (102) that allows the frequenciesof 49.670-49.990 MHz to pass. The filter is connected to a receiveamplifier (103) that amplifies the filtered signal by approximately 18dB. The output of the amplifier is then filtered by another bandpassfilter (104) before being input to the receiver (105). In the preferredembodiment, the receiver is an MC3363 available from Motorola, Inc.

The receiver (105) mixes the signal down to 10.7 MHz and then to 455kHz. This signal is then demodulated. The local oscillators in thereceiver (105) are controlled by a phase locked loop (120) whosefrequency is controlled by a microprocessor (112). The microprocessor(112) also receives a carrier detect signal, labeled RSSI in FIG. 1, andthe demodulated signal from the receiver (105), labeled RX Data. Thedemodulated signal is used by the microprocessor (112) to interpretcoded data messages transmitted by the remote unit. These coded datamessages are used for DTMF dialing, telephone hook switch control, andcontrol of the scrambling circuit. The carrier detect signal is used todetect the presence of a transmitter occupying the base unit receivechannel. In the preferred embodiment, the microprocessor (112) is anMC68HCL05C4 available from Motorola, Inc.

The output of the receiver (105) is filtered again by another bandpassfilter (106). The output of the filter (106) is input to the scramblingcircuit (107). In the preferred embodiment, the scrambling circuit (107)is an MX108DW Full-Duplex Voiceband Scrambler integrated circuitavailable from MX.Com, Inc.

The unscrambled output of the scrambler circuit (107) is filtered in ade-emphasis filter (108) before being input to an expander (109). Theexpansion process applies gain to the input signal that is varied as afunction of the signal magnitude. The effective gain being greater forlarge magnitude signals and less for small magnitude signals. After thissignal has been filtered by a highpass filter (110), it is input to aspeakerphone circuit (111) and is also output to the land-line telephonesystem. If the speakerphone is being used, such as in a three way callbetween the base, the remote, and a land-line party, the audio signal isoutput to the base unit's speaker.

The transmit path of the base is comprised of the audio signal, eitherfrom the microphone (121) of the speakerphone or the land-line party,being bandpass filtered (113) before going through a compressor (114).The compressor (114) applies gain to the audio signal that is varied asa function of the signal magnitude, the effective gain being greater forsmall signals. An example of a compandor (compressor/expander) isMC33110 manufactured by Motorola, Inc. The dynamic range of thecompandor is 80 dB.

The output of the compressor (114) is input to a pre-emphasis filter(115) before being scrambled by the scrambler circuit (116). Thescrambler circuit (116) is the second half of the same integratedcircuit used to unscramble the received signal. The scrambled audiosignal is bandpass filtered (117) before being modulated by thetransmitter (118) to a frequency in the range of 46.610-46.970 MHz. Theactual frequency used by the telephone can be user selectable by aswitch or other control on the telephone base or remote. Both thereceive and transmit frequency ranges are set by the FCC for allcordless telephones. An example of a transmitter is MC2833 manufacturedby Motorola, Inc.

The output of the transmitter is bandpass filtered (119) to filter outthe signal outside the 46.610-46.970 MHz range. This signal is thentransmitted from the antenna (101) to the remote unit.

The remote unit is comprised of the antenna (201) that receives thesignal from the base unit. A bandpass filter (202) removes that part ofthe signal outside the range of 46.610-46.970 MHz. A gain ofapproximately 18 dB is then applied to the signal by a receive amplifier(203) before it is bandpass filtered (204) again. This signal is theninput to the receiver (205).

The remote unit receiver (205) mixes the signal down to 10.7 MHz andthen to 455 kHz. This signal is then demodulated. The demodulatedsignal, labeled RX Data, is input to the microprocessor (212) tointerpret the coded data messages transmitted by the base unit. Thesecoded data messages are used to control the remote ringer and thescrambling circuit.

The demodulated signal is also bandpass filtered (206) before beinginput to the scrambler circuit (207) for descrambling. The scramblercircuit (207) is identical to the integrated circuit used in the baseunit.

The unscrambled output is filtered by a de-emphasis filter (208) beforegoing through an expander (209). The expanded signal is bandpassfiltered (210) and amplified (211). The amplifier (211) is controlled bythe microprocessor (212). By operating a control on the remote unit, theuser can increase or decrease the speaker (220) volume by increasing ordecreasing the gain applied to the signal at this point. The amplifiedsignal is input to the earpiece speaker of the remote unit.

The microphone (221) of the remote generates the audio signal that isbandpass filtered (213), compressed (214), and filtered by apre-emphasis filter (215) before being processed by the scramblercircuit (216). This circuit (216) is the second half of the integratedcircuit used to unscramble the received signal.

The scrambled signal is bandpass filtered (217) before being modulatedto a frequency in the range of 49.67-49.99 MHz by the transmitter (218).The modulated signal is bandpass filtered (219) to remove the signaloutside this frequency range and is transmitted on the remote's antenna(201).

The keypad on the remote (222) is used to take the telephone off hook,enter telephone numbers, change the frequency used to communicate withthe base, and to enable the scramble demonstration function. The keypadenables the scrambling demonstration function by telling themicroprocessor to generate a control signal to the scrambling integratedcircuit. In the preferred embodiment, the base unit also has a keypad.

The control signals in both the remote and the base units are connectedto the transmitter after the scrambling function so that these signalsare not scrambled. The control signals, generated by the microprocessor,are wire-ORed with the scrambled voice signal to the transmitter input.

The scrambling of the signal between the remote and the base isoperating continuously. This scrambling, however, is not evident to thetelephone user or the party on the other end of the call. It would notbe known, therefore, if the scrambling was operating properly. Thescrambling demonstration process of the present invention provides bothparties to the telephone call the ability to hear the scrambled versionof the signal.

The scrambling demonstration process is illustrated in FIG. 3. While thetelephone is in a call, the keypad is scanned (301) by themicroprocessor for the depression of a key, such as a Secure Demo key,that indicates that the remote unit user wishes to enter the scramblingdemonstration mode. In the preferred embodiment, this mode is referredto as the Secure Demo mode.

If the key is depressed (302), a timer in the microprocessor is started(303). If the key is released immediately (304), the timer is cleared(305) and the keypad scanned again (301). If the key is still depressed(304), the timer is incremented (306) and checked to determine if thekey has been depressed for at least 3 seconds (307). This time check iscontinued until the key has been depressed for 3 seconds. At this point,the scrambling demonstration mode is operating (308). This can beindicated on the remote or the base by flashing indicator lights (309).The remote keypad may be used for the above process or, if the base hasa keypad, this keypad may be used to enter the scrambling demonstrationmode.

The block diagram of the MX108DW Full-Duplex Voiceband Scramblerintegrated circuit is illustrated in FIG. 4. Since this integratedcircuit uses frequency inversion scrambling, a signal that is input tothe circuit that has already been scrambled will be returned to normalby the circuit.

In order to put the telephone into the scrambling demonstration mode,the MX108DW must be able to transmit the input signal withoutscrambling. This is accomplished by the control input labeledClear/Scramble. By applying a logic one to this input, the signal istransmitted through the path via the clear path. A logic zero on thiscontrol input will cause the signal to be transmitted through the pathvia the scramble path.

The preferred embodiment of the present invention disables the entirescrambling integrated circuit in the remote unit when the scramblingdemonstration mode is initiated. This allows the scrambled audio signalto be heard by both the telephone user and the land-line party to theconversation.

In an alternate embodiment, the scrambling circuits in the receive pathsof both the base and remote units are disabled in the scramblingdemonstration mode. This embodiment, however, requires that a command besent by the unit that is initiating the scrambling demonstration mode tothe other unit, instructing the receiving unit to disable the receivepath scrambling circuit. This command can be transmitted during block(308) of FIG. 3.

In another embodiment of the present invention, the base speakerphonecan also have a scrambling circuit. This allows a conversation betweenthe speakerphone and the land-line party to also use the scramblingdemonatration mode. This can be enabled in the same manner isillustrated in FIG. 3.

The preferred embodiment disables the scrambling circuits until any keyis depressed, causing the disabled scrambling circuits to be enabled.Alternate embodiments can disable the scrambling circuits for apredetermined time. The scrambling circuits can also be enabled, andthus the scrambling demonstration mode ended, upon hang-up of the callin progress.

In summary, a wireless or cordless telephone having controllablescrambling between the remote unit and the base unit has been shown,thus providing secure conversations between the telephone user and theland-line party. Using the present invention, telephone user can stillenjoy the convenience of a cordless telephone without unwanted partieslistening to the conversation. Additionally, the present inventionallows the scrambled signal to be heard by each party, in ademonstration mode, to determine that the scrambling circuitry isoperating properly.

We claim:
 1. A wireless communication system having a base unit, coupledto a land-line telephone system, and a remote unit, the remote unitcommunicating with the base unit over a communication medium, the systemcomprising:first frequency inversion encryption means, located in atransmit path of the remote unit, for encrypting voice signalstransmitted to the base unit; first frequency inversion decryptionmeans, located in a receive path of the remote unit, for decryptingvoice signals received from the base unit; second frequency inversionencryption means, located in a transmit path of the base unit, forencrypting voice signals transmitted to the remote unit; secondfrequency inversion decryption means, located in a receive path of thebase unit, for decrypting voice signals received from the remote unit;and control entry means for generating a control signal to disable thefirst and second frequency inversion decryption means.
 2. The system ofclaim 1 wherein the first and second frequency inversion decryptionmeans are disabled for a predetermined time.
 3. The system of claim 1wherein the first and second frequency inversion decryption means aredisabled until the control entry means generates a control signal toenable them.
 4. The system of claim 1 wherein the first and secondfrequecy inversion decryption means are enabled when an ongoingtelephone call is terminated.
 5. A wireless communication system,comprising:a base unit having a receive path and a transmit path, thereceive path comprising: first receiving means, for receiving voice andcontrol signals and first frequency inversion decryption means fordecrypting the voice signals, the transmit path comprising: firsttransmitting means for transmitting voice signals and first frequencyinversion encryption means for encrypting the voice signals; a remoteunit that communicates over radio frequencies with the base unit, theremote unit having a receive path and a transmit path, the receive pathcomprising: second receiving means, for receiving the voice signals andsecond frequency inversion decryption means for decrypting the voicesignals, the transmit path comprising: electro-audio transducer meansfor generating voice signals, second frequency inversion encryptionmeans for encrypting the voice signals, and second transmitting meansfor transmitting the voice and control signals to the base unit; andcontrol entry means for generating the control signals that disable thefirst and second frequency inversion decryption means.
 6. The system ofclaim 5 wherein the control entry means is located in the remote unit.7. The system of claim 5 wherein the control entry means is located inthe base unit.
 8. The system of claim 5 wherein both the remote unit andthe base unit contain control entry means.
 9. A wireless communicationsystem having a base unit, coupled to a land-line telephone system, anda remote unit that communicates with the base unit over radiofrequencies, the system comprising:first frequency inversion encryptionmeans, located in a transmit path of the remote unit, for encryptingvoice signals transmitted to the base unit; first frequency inversiondecryption means, located in a receive path of the remote unit, fordecrypting voice signals received from the base unit; second frequencyinversion encryption means, located in a transmit path of the base unit,for encrypting voice signals transmitted to the remote unit; p1 secondfrequency inversion decryption means, located in a receive path of thebase unit, for decrypting voice signals received from the remote unit;and control entry means, located in the remote unit, for generating acontrol signal to disable the first frequency inversion decryption meansand the first frequency inversion encryption means.
 10. A method forenabling a scrambling demonstration mode in a communications devicehaving a base unit and a remote unit, the base unit coupled to aland-line telephone system, the base and remote units communicating witheach other by frequency inversion scrambled voice signals transmitted atradio frequencies, at least the remote unit having a keypad, the methodcomprising the steps of:scanning the keypad for the activation of ascrambling demonstration key; and is the scrambling demonstration key isactivated for a predetermined time, enabling the scramblingdemonstration mode to allow the scrambled voice signals to be heard atleast by a user of the remote unit and a user of the land-line telephonesystem.
 11. The method of claim 10 wherein the predetermined time is 3seconds.